Valve positioner with overpressure protection capabilities

ABSTRACT

A method of limiting control pressure provided to an actuator of a valve coupled to a valve positioner, a drive signal is provided to a pneumatic stage of the valve positioner. The pneumatic stage is arranged to control output pressure of the valve positioner in accordance with the drive signal. A pressure measurement from a pressure sensor communicatively coupled to the valve positioner is obtained, and an abnormal pressure is detected based on the pressure measurement. In response to detecting the abnormal pressure, the drive signal is controlled so as to limit the output pressure of the valve positioner, wherein the output pressure provides control pressure to the actuator.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 14/151,585filed Jan. 9, 2014, entitled “Valve Positioner with OverpressureProtection Capabilities,” the entire disclosure of which is herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to process control systems and,more particularly, to providing overpressure protection for processcontrol devices in process control systems.

DESCRIPTION OF THE RELATED ART

Process control systems, such as distributed or scalable process controlsystems like those used in chemical, petroleum or other processes,typically include one or more process controllers communicativelycoupled to one or more field devices via analog, digital, or combinedanalog/digital buses. The field devices, which may include, for example,control valve assemblies (e.g., control valves, actuators, valvecontrollers), valve positioners, switches, and transmitters (e.g.,temperature, pressure and flow rate sensors), perform functions withinthe process such as opening or closing valves, measuring processparameters, and performing basic diagnostics. The process controllerreceives signals indicative of process measurements made by the fielddevices and/or other information pertaining to the field devices, anduses this information to execute or implement one or more controlroutines to generate control signals, which are sent over the buses tothe field devices to control the operation of the process. Informationfrom each of the field devices and the controller is typically madeavailable to one or more applications executed by one or more otherhardware devices, such as host or user workstations, personal computersor computing devices, to enable an operator to perform any desiredfunction regarding the process, such as setting parameters for theprocess, viewing the current state of the process, modifying theoperation of the process, etc.

Process control systems often employ electro-pneumatic controllers(e.g., electro-pneumatic positioners) to control process control devicesoperating in the process control systems (e.g., control valves, pumps,dampers, etc.). Electro-pneumatic controllers are usually configured toreceive one or more control signals and convert those control signalsinto a pressure provided to a pneumatic actuator to cause a desiredoperation of the process control device coupled to the pneumaticactuator. For example, if a process control routine requires apneumatically-actuated valve to pass a greater volume of a processfluid, the magnitude of the control signal applied to anelectro-pneumatic controller associated with the valve may be increased(e.g., from 10 milliamps (mA) to 15 mA in a case where theelectro-pneumatic controller is configured to receive a 4-20 mA controlsignal).

An electro-pneumatic controller typically includes a pneumatic module,which may include a first pneumatic stage, such as a current to pressure(I/P) transducer or voltage to pressure (E/P) transducer, and a secondpneumatic stage, such as a relay. The pneumatic module typicallyreceives a pressurized supply fluid, such as air, and modulates thepressurized supply fluid in accordance with a control signal (e.g., acurrent drive signal) to produce a pneumatic output signal in responseto the control signal. The supply fluid is typically supplied to theelectro-pneumatic controller and, more specifically, to the pneumaticstage of the electro-pneumatic controller, via a supply pressureregulator, such as an airset device or an air filter device, providedbetween a pressure source and a pressure supply input of theelectro-pneumatic device. The pressure regulator is typically arrangedto provide a suitable supply pressure that ensures that the controlpressure output of the elector-pneumatic controller does not exceed acertain maximum pressure (e.g., a maximum control pressure rating of anactuator being controlled by the electro-pneumatic controller), therebyproviding overpressure protection for the device being controlled.

A failure or a malfunction of the pressure regulator, however, mayresult in over-pressuring the device being controlled by theelectro-pneumatic controller, which may damage the device beingcontrolled (e.g. rupture an actuator) and may lead to a potentiallydangerous situation within the process control system. To protect thedevice in case of a failure or a malfunction of the pressure regulator,a relief valve is often coupled between the control pressure output ofthe electro-pneumatic controller and a control pressure input of thedevice being controlled by the controller. The relief valve bleedscontrol fluid to, for example, the atmosphere when the pressure of thecontrol fluid increases due, for example, to failure or malfunction ofthe pressure regulator device. As such, the relief valve providesredundant overpressure protection to the device to avoidover-pressurizing the device in case of a failure or a malfunction ofthe supply fluid regulator device. However, such relief valves may beexpensive and inconvenient and/or may be difficult to install in processcontrol devices.

SUMMARY

In accordance with a first exemplary aspect, a method of limitingcontrol pressure provided to an actuator of a valve coupled to a valvepositioner. The method includes providing a drive signal to a pneumaticstage of the valve positioner, wherein the pneumatic stage is arrangedto control output pressure of the valve positioner in accordance withthe drive signal. The method also includes obtaining a pressuremeasurement from a pressure sensor communicatively coupled to the valvepositioner, and detecting an abnormal pressure based on the pressuremeasurement. The method further includes in response to detecting theabnormal pressure, controlling the drive signal so as to limit theoutput pressure of the valve positioner, wherein the output pressureprovides control pressure to the actuator.

In accordance with a second exemplary aspect, a process control devicecomprises a valve, an actuator coupled to the valve and configured tocontrol a position of the valve, and a valve positioner coupled to thevalve and to the actuator and configured to provide a control pressureto the actuator to control a position of the actuator. The valvepositioner comprises a pneumatic stage arranged to receive a drivesignal and to control output pressure of the valve positioner inaccordance with the drive signal. The valve positioner additionallycomprises an overpressure protection module configured to obtain ameasurement from a pressure sensor communicatively coupled to the valvepositioner, detect an abnormal pressure based on the pressuremeasurement, and in response to detecting the abnormal pressure, controlthe drive signal so as to limit the output pressure of the valvepositioner, wherein the output pressure provides the control pressure tothe actuator.

In accordance with a third exemplary aspect, a valve positioner coupledto a process control device comprising a valve and an actuator, thevalve positioner configured to receive a control signal from a processcontrol system and to control a pressure supplied to the actuator inaccordance with the control signal. The valve positioner comprises apneumatic stage arranged to receive a drive signal and to control outputpressure of the valve positioner in accordance with the drive signal.The overpressure protection module additionally comprises anoverpressure protection module configured to obtain a measurement from apressure sensor communicatively coupled to the valve positioner, detectan abnormal pressure based on the pressure measurement, and in responseto detecting the abnormal pressure, control the drive signal so as tolimit the output pressure of the valve positioner, wherein the outputpressure provides the control pressure to the actuator.

In further accordance with any one or more of the forgoing first,second, or third aspects, a method, a process control device and/or avalve positioner may further include any one or more of the followingpreferred forms.

In one preferred form, the pressure sensor is configured to sense alevel of a supply pressure provided to the valve positioner.

In another preferred form, the pressure sensor is configured to sense alevel of the output pressure of the valve positioner.

In another preferred form, detecting the abnormal pressure comprisescomparing the pressure measurement to a predetermined threshold, anddetermining that the pressure is abnormal when the measured pressureexceeds the predetermined threshold.

In another preferred form, the valve positioner includes a processor anda memory, and detecting the abnormal pressure and controlling the drivesignal comprises executing computer readable instructions stored in thememory.

In another preferred form the valve positioner includes a controlcircuit configured to receive the pressure measurement, and detectingthe abnormal pressure and controlling the drive signal is performed bythe control circuit.

In another preferred form the drive signal is a current signal, andcontrolling the drive signal comprises setting the drive signal to avalue at or near zero milliamperes.

In another preferred form the drive signal is a voltage signal, andcontrolling the drive signal comprises setting the drive signal to avalue at or near zero millivolts.

In another preferred form, the overpressure protection module isconfigured to compare the pressure measurement to a predeterminedthreshold, and determine that the pressure is abnormal when the measuredpressure exceeds the predetermined threshold.

In another preferred form, the valve positioner includes a processor anda memory, and the overpressure protection module comprises computerreadable instructions stored in the memory and executable by theprocessor.

In another preferred form, the overpressure protection module comprisesa hardware control circuit.

In another preferred form, the drive signal is a current drive signal,and the overpressure detection module is configured to, in response todetecting the abnormal pressure, set the drive signal to zero to a valueat or near zero milliamperes.

In another preferred form, the drive signal is a voltage drive signal,and the overpressure detection module is configured, in response todetecting the abnormal pressure, set the drive signal to a value at ornear zero millivolts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a process control system havingone or more field devices arranged in accordance with the principles ofthe present disclosure.

FIG. 2 is a block diagram of an example field device arranged inaccordance with an embodiment of the present disclosure.

FIG. 3 is a block diagram of an example field device arranged inaccordance with another embodiment of the present disclosure.

FIG. 4 is a flow chart of an overpressure protection scheme according toan embodiment.

FIG. 5 is a block diagram of an example field device arranged inaccordance with yet another embodiment of the present disclosure.

FIG. 6 is a block diagram of an example field device arranged inaccordance with still another embodiment of the present disclosure.

FIG. 7 is a flow chart of an exemplary method for limiting controlpressure provided to an actuator of a valve coupled to a valvepositioner.

DETAILED DESCRIPTION

Referring now to FIG. 1, a process control system 10 constructed inaccordance with one version of the present disclosure is depictedincorporating one or more field devices 15, 16, 17, 18, 19, 20, 21, 22,and 71 in communication with a process controller 11, which in turn, isin communication with a data historian 12 and one or more userworkstations 13, each having a display screen 14. So configured, thecontroller 11 delivers signals to and receives signals from the fielddevices 15, 16, 17, 18, 19, 20, 21, 22, and 71 and the workstations 13to control the process control system.

In additional detail, the process controller 11 of the process controlsystem 10 of the version depicted in FIG. 1 is connected via hardwiredcommunication connections to field devices 15, 16, 17, 18, 19, 20, 21,and 22 via input/output (I/O) cards 26 and 28. The data historian 12 maybe any desired type of data collection unit having any desired type ofmemory and any desired or known software, hardware or firmware forstoring data. Moreover, while the data historian 12 is illustrated as aseparate device in FIG. 1, it may instead or in addition be part of oneof the workstations 13 or another computer device, such as a server. Thecontroller 11, which may be, by way of example, a DeltaV™ controllersold by Emerson Process Management, is communicatively connected to theworkstations 13 and to the data historian 12 via a communication network29 which may be, for example, an Ethernet connection.

As mentioned, the controller 11 is illustrated as being communicativelyconnected to the field devices 15, 16, 17, 18, 19, 20, 21, and 22 usinga hardwired communication scheme which may include the use of anydesired hardware, software and/or firmware to implement hardwiredcommunications, including, for example, standard 4-20 mA communications,and/or any communications using any smart communication protocol such asthe FOUNDATION® Fieldbus communication protocol, the HART® communicationprotocol, etc. The field devices 15, 16, 17, 18, 19, 20, 21, and 22 maybe any types of devices, such as sensors, control valve assemblies,transmitters, positioners, etc., while the I/O cards 26 and 28 may beany types of I/O devices conforming to any desired communication orcontroller protocol. In the embodiment illustrated in FIG. 1, the fielddevices 15, 16, 17, 18 are standard 4-20 mA devices that communicateover analog lines to the I/O card 26, while the digital field devices19, 20, 21, 22 can be smart devices, such as HART® communicating devicesand Fieldbus field devices, that communicate over a digital bus to theI/O card 28 using Fieldbus protocol communications. Of course, the fielddevices 15, 16, 17, 18, 19, 20, 21, and 22 may conform to any otherdesired standard(s) or protocols, including any standards or protocolsdeveloped in the future.

In addition, the process control system 10 depicted in FIG. 1 includes anumber of wireless field devices 60, 61, 62, 63, 64 and 71 disposed inthe plant to be controlled. The field devices 60, 61, 62, 63, 64 aredepicted as transmitters (e.g., process variable sensors) while thefield device 71 is depicted as a control valve assembly including, forexample, a control valve and an actuator. Wireless communications may beestablished between the controller 11 and the field devices 60, 61, 62,63, 64 and 71 using any desired wireless communication equipment,including hardware, software, firmware, or any combination thereof nowknown or later developed. In the version illustrated in FIG. 1, anantenna 65 is coupled to and is dedicated to perform wirelesscommunications for the transmitter 60, while a wireless router or othermodule 66 having an antenna 67 is coupled to collectively handlewireless communications for the transmitters 61, 62, 63, and 64.Likewise, an antenna 72 is coupled to the control valve assembly 71 toperform wireless communications for the control valve assembly 71. Thefield devices or associated hardware 60, 61, 62, 63, 64, 66 and 71 mayimplement protocol stack operations used by an appropriate wirelesscommunication protocol to receive, decode, route, encode and sendwireless signals via the antennas 65, 67 and 72 to implement wirelesscommunications between the process controller 11 and the transmitters60, 61, 62, 63, 64 and the control valve assembly 71.

If desired, the transmitters 60, 61, 62, 63, 64 can constitute the solelink between various process sensors (transmitters) and the processcontroller 11 and, as such, are relied upon to send accurate signals tothe controller 11 to ensure that process performance is not compromised.The transmitters 60, 61, 62, 63, 64, often referred to as processvariable transmitters (PVTs), therefore may play a significant role inthe control of the overall control process. Additionally, the controlvalve assembly 71 may provide measurements made by sensors within thecontrol valve assembly 71 or may provide other data generated by orcomputed by the control valve assembly 71 to the controller 11 as partof its operation. Of course, as is known, the control valve assembly 71may also receive control signals from the controller 11 to effectphysical parameters, e.g., flow, within the overall process.

The process controller 11 is coupled to one or more I/O devices 73 and74, each connected to a respective antenna 75 and 76, and these I/Odevices and antennas 73, 74, 75, 76 operate as transmitters/receivers toperform wireless communications with the wireless field devices 61, 62,63, 64 and 71 via one or more wireless communication networks. Thewireless communications between the field devices (e.g., thetransmitters 60, 61, 62, 63, 64 and the control valve assembly 71) maybe performed using one or more known wireless communication protocols,such as the WirelessHART® protocol, the Ember protocol, a WiFi protocol,an IEEE wireless standard, etc. Still further, the I/O devices 73 and 74may implement protocol stack operations used by these communicationprotocols to receive, decode, route, encode and send wireless signalsvia the antennas 75 and 76 to implement wireless communications betweenthe controller 11 and the transmitters 60, 61, 62, 63, 64 and thecontrol valve assembly 71.

As illustrated in FIG. 1, the controller 11 conventionally includes aprocessor 77 that implements or oversees one or more process controlroutines (or any module, block, or sub-routine thereof) stored in amemory 78. The process control routines stored in the memory 78 mayinclude or be associated with control loops being implemented within theprocess plant. Generally speaking, and as is generally known, theprocess controller 11 executes one or more control routines andcommunicates with the field devices 15, 16, 17, 18, 19, 20, 21, 22, 60,61, 62, 63, 64, and 71, the user workstations 13 and the data historian12 to control a process in any desired manner(s).

FIG. 2 is a block diagram of an example field device 200 arranged inaccordance with an embodiment of the present disclosure. The fielddevice 200 may be incorporated within a process control system such asthe example process control system 100 of FIG. 1. With reference to FIG.1 the field device 200 may be one of the field devices 15-18 whichcommunicates with the controller 11 over an analog connection usingstandard 4-20 mA communication, for example. In another embodiment, thefield device 200 may be one of the field devices 19-22 whichcommunicates with the controller 11 over a digital bus using a digitalcommunication protocol, such as a HART or a fieldbus protocol, or anyother suitable digital communications protocol. In still anotherembodiment, the field device 200 may be the field device 72 whichcommunicates with the controller 11 via a wireless connection using anysuitable wireless communication protocol. In this embodiment, the fielddevice 200 includes an antenna (not shown) included in or coupled to thefield device 200 to enable wireless communication between the fielddevice 200 and the controller 11.

The field device 200 is illustrated in FIG. 2 as a control valveassembly having a valve 202, an actuator 204, and a valve positioner 206communicatively coupled to the valve 202 and the actuator 204. The valve202 may be, for example, a rotary valve, a quarter-turn valve, a damper,or any other control device or apparatus. The actuator 204 may be apneumatic actuator operatively coupled to a flow control member withinthe valve 202 via a valve stem, for example. The valve stem may moveflow control member in a first direction (e.g., away from the valveseat) to allow fluid flow between the inlet and the outlet and in asecond direction (e.g., toward the valve seat) to restrict or preventfluid flow between the inlet and the outlet. In various embodiments, theactuator 204 may include a double-acting piston actuator, asingle-acting spring return diaphragm or piston actuator, or any othersuitable actuator or process control device.

In FIG. 2, the valve positioner 206 is illustrated as a digital valvepositioner having a processor 208, a memory 210, and an interface module212. Additionally, the valve positioner 206 includes a pneumatic module214 having a first pneumatic stage 215 and a second pneumatic stage 216.The first pneumatic stage 215 may be an electro pneumatic transducer,such as a current to pressure (I/P) transducer, a voltage to pressure(E/P) transducer, etc., that may generate an output pressure proportionto a drive signal provided to the first pneumatic stage 215. The secondpneumatic stage 216 may operate to amplify the pressure generated by thefirst pneumatic stage 215 to produce a pressure suitable for operationof the actuator 204. The second pneumatic stage 216 may be, for example,a spool valve, a poppet valve, a relay, etc. The network interface 212of the valve positioner 206 is configured to transmit and/or receivesignals according to a particular communication protocol of the processcontrol system of which the field device 200 is a part. In someembodiments, the communication protocol is a wireless mesh networkprotocol, such as the WirelessHART or ISA 100.11a protocol,Alternatively, the network interface 212 may support wiredcommunications, such as standard 4-20 mA communications, and/or anycommunications using any smart communication protocol such as theFOUNDATION® Fieldbus communication protocol, the HART® communicationprotocol, etc. In some embodiments, the network interface 212 includes atransceiver (not shown). The transceiver typically includes one or moreprocessors (also not shown) for executing instructions relating tophysical (PHY) layer and other layer (e.g., medium access control (MAC)layer) tasks according to the wireless communication protocol utilizedby the process control system. The network interface may be coupled toone or more antennas (not shown). Via the one or more antennas, thenetwork interface 212 transmits and/or receives data packets accordingto the wireless communication protocol. The network interface 212 ispreferably configured to both transmit and receive data packets.

The processor 208 may be a general purpose processor, a digital signalprocessor, an ASIC, field programmable gate array, or any other know orlater developed processor. The processor 208 operates pursuant toinstructions stored in the memory 210. While the example field device200 of FIG. 2 includes one processor 208, other embodiments may includetwo or more processors that perform the functions of the processor 208.The memory 210 may be a volatile memory or a non-volatile memory. Thememory 210 may include one or more of a read-only memory (ROM),random-access memory (RAM), a flash memory, an electronic erasableprogram read-only memory (EEPROM), or other type of memory. The memory210 may include an optical, magnetic (hard drive), or any other form ofdata storage device.

In operation, the processor 208 receives a command signal, such as a 4to 20 mA command signal or a 0 to 10 V command signal, that represents adesired position of the valve 202. The processor 208 also receives anindication of an actual position of the valve 202 provided to theprocessor 208 by a travel sensor 218. The travel sensor 218 may be ananalog travel sensor and may be coupled to the processor 208 via ananalog to digital converter 219. The analog to digital converter 219 mayconvert the analog signal produced by the travel sensor 218 to a digitalsignal suitable for use by the processor 208. In another embodiment, thetravel sensor 218 may be a digital sensor. For example, the travelsensor 218 may include an analog to digital converter internal to thetravel sensor 218. In this case, the analog to digital converter 219 maybe omitted and the output of the travel sensor 218 may be provideddirectly to the processor 208.

The processor 208 compares the desired position for the valve 202indicated by the command signal received from the process controllerwith the actual position of the valve 202 indicated by the travel sensor218, and generates a drive signal for the pneumatic module 214 based ona difference between the desired position and the actual position of thevalve 202. The drive signal may be a current drive signal or a voltagedrive signal, for example. The drive signal corresponds to an amount thevalve positioner 206 is to change the position of the actuator 204coupled to the valve 202. The drive signal generated by the processor208 is provided to the first pneumatic stage 215 of the pneumatic module214 via a digital to analog converter 217, which converts the (digital)drive signal generated by the processor 208 to an analog drive signalsuitable for driving the first pneumatic stage 215. The first pneumaticstage 215 modulates a pressurized supply fluid supplied to firstpneumatic stage 215 in accordance with the drive signal to produce anoutput pressure that is proportional to the drive signal. The outputpressure of the first pneumatic stage 215 is provided to the secondpneumatic stage 216 which may amplify the pressure output of the firstpneumatic stage 215 and may provide the amplified pressure to thepressure output of the valve positioner 206. The pressure output of thevalve positioner 206 is coupled to a control pressure input of theactuator 204 and provides the control pressure for the actuator 204 tocontrol the position of the actuator 204, thereby controlling the valve202 to move towards the desired position for the valve 202.

It should be noted that while the first pneumatic stage 215 is generallydescribed herein as being a proportional I/P transducer, the firstpneumatic stage 215 may instead be an on/off transducer. In this case,the pneumatic module 214 may alternate between providing the pressurizedsupply fluid to the control pressure input of the actuator 204, andexhausting the pressurized supply fluid (e.g., to the atmosphere),thereby controlling the position of the actuator 204. It is also notedthat the valve positioner 206 may include other type of position controlmechanisms instead of or in addition to those illustrated in FIG. 2.Further, it should be understood that the field device 200 may be anyother type of pneumatically controlled device operating within a processcontrol system. For example, the field device 200 may be a damper, etc.

With continued reference to FIG. 2, the supply pressure may be providedto the valve positioner 206 and, more particularly, to the firstpneumatic stage 215 and the second pneumatic stage 216, via a pressureregulator, such as an airset 220. The airset 220 may regulate and filtera pressurized supply fluid, such as air, provided by a pressure supplysource within the process control system, and may reduce the pressureprovided by the pressure supply source to a pressure level suitable foruse by the valve positioner 206 and by the actuator 204. Generally, thevalve positioner 206 produces output pressure by modulating the supplypressure, and the produced output pressure is typically pressurized at alevel lower than the supply pressure. In some situations, the valvepositioner 206 may output the full supply pressure provided to the valvepositioner 206 to the actuator 204 so as to provide a maximum force tothe actuator 204, for example to force the valve 202 into a valve seat.By regulating and/or reducing the pressure provided by the pressuresupply source, the pressure regulator 220 typically ensures that theoutput control pressure of the valve positioner 206 does not exceed acertain a maximum level, such as a maximum pressure rating of theactuator 204, thereby providing overpressure protection for the actuator204. However, in a case of a failure or a malfunction of the airset 220(e.g., when the airset 220 is stuck open and, accordingly, supplies afull, rather than reduced, supply pressure to the valve positioner 206),the valve positioner 206 may produce a pressure output that exceeds amaximum rating of the actuator 204 or other desired maximum controlpressure level for the actuator 204, for example in a situation in whichthe valve positioner 206 outputs the full supply pressure of the valvepositioner 206 to the actuator 204, thereby exceeding the maximumcontrol pressure of the actuator 204. Exceeding the maximum pressure ofthe actuator 204 may over-pressurize the actuator 204, which may damagethe actuator 204 and/or may lead to a potentially dangerous situation inthe process control system.

The valve positioner 206 includes an overpressure protection module 222and a pressure sensor 224. The overpressure module 222 generally ensuresthat a safe control pressure is provided to the actuator 204 even incases of a failure or a malfunction of the airset 220, thereby providingadditional or redundant overpressure protection for the actuator 204. Inthe embodiment of FIG. 2, the overpressure protection module 222comprises computer readable instructions stored in the memory 210. Theprocessor 208 is configured to execute the computer readableinstructions to provide overpressure protection for the actuator 204.The overpressure protection module 222 may operate to limit the drivesignal provided to the first pneumatic stage 215 to ensure that thepressure output of the valve positioner 206 does not exceed a certainmaximum value, such as the maximum pressure rating of the actuator 204,or any other suitable value required or desired for operation of theactuator 204.

The pressure sensor 224 is coupled to the output pressure of the valvepositioner 206 and is configured to provide measurements of the outputpressure of the valve positioner 206 to the processor 208. The pressuresensor 224 may be an analog pressure sensor, in which case the output ofthe pressure sensor 224 may be coupled to an analog to digital converter225. The analog to digital converter 225 may convert the analog signalproduced by the pressure sensor 224 to a digital signal suitable for useby the processor 208. In another embodiment, the pressure sensor 224 maybe a digital pressure sensor. For example, the pressure sensor 224 mayinclude an analog to digital converter internal to the pressure sensor224. In this case, the analog to digital converter 225 may be omittedand the output of the pressure sensor 224 may be provided directly tothe processor 208.

The overpressure protection module 222 may monitor the pressure outputof the valve positioner 206 by periodically obtaining pressuremeasurements provided by the pressure sensor 224. The overpressureprotection module 222 may compare the measurements obtained from thepressure sensor 224 to a predetermined threshold, and may detect anabnormal (e.g., abnormally high) pressure when the measured pressureexceeds the predetermined threshold. In response to detecting theabnormal pressure, the overpressure protection module 222 may controlthe level of the drive signal supplied by the processor 208 to the firstpneumatic stage 215 so as to limit the output pressure of the firstpneumatic stage 215 and, accordingly, limit the output pressure of thevalve positioner 206. For example, the overpressure protection module222 may set the drive signal to a predetermined value, such as a valueat or near zero milliamps in the case that the drive signal is a currentdrive signal, or a value at or near zero millivolts in the case that thedrive signal is a voltage drive signal. Alternatively, the overpressureprotection module 222 may set the drive signal to another suitablevalue, or may adjust the drive signal in another suitable manner, so asto reduce or limit the pressure output of the valve positioner 206.

As yet another example, in response to detecting the abnormal pressure,the overpressure protection module 222 may operate to prevent anyfurther adjustments of the drive signal so as to stop any furtheradjustments of the output pressure level of the valve positioner 206. Inthis case, the output pressure level of the valve positioner 206 willstop responding to further changes in the command signal received by thevalve positioner 206. As such, the output pressure of the valvepositioner 206 will remain at a level produced by the valve positioner206 prior to detection of the abnormal pressure, such as prior tomalfunction or failure of the airset 220, in this embodiment.Alternatively, in response to detecting the abnormal pressure, theoverpressure protection module 222 may operate to prevent any furtherincreases in the level of the drive signal, while still allowing thelevel of the drive signal to decrease in response to receiving a commandsignal that results in a decrease of the dive signal level. In anyevent, the overpressure protection module 222, in response to detectingthe abnormal pressure, operates to ensure that the output pressure leveldoes not increase to a level that would be unsafe and/or undesired foroperation of the actuator 204.

The components of the valve positioner 206 may be communicativelycoupled as illustrated in FIG. 2 or may be coupled in any other suitablemanner. Further, the valve positioner 206 may include any othercomponents for controlling and/or providing pressure to the actuator 204in addition to or instead of the components illustrated in FIG. 2.Additionally or alternatively, although not shown, the valve positioner206 may include other signal processing components such as, for example,analog-to-digital converters, digital-to-analog converters, filters(e.g., low-pass filters, high-pass filters, and digital filters),amplifiers, etc.

FIG. 3 is a block diagram of the field device 200 arranged in accordancewith another embodiment of the present disclosure. In the embodiment ofFIG. 3, the valve positioner 206 of FIG. 2 is replaced by a valvepositioner 306. The valve positioner 306 is generally similar to thevalve positioner 206 and includes many like-numbered elements to thevalve positioner 206. However, the valve positioner 306 is configured todetect an abnormal supply pressure provided to the valve positioner 306rather than detecting an abnormal output pressure as is the case withthe valve positioner 206.

The valve positioner 306 includes an overpressure protection module 322and a pressure sensor 324. The pressure sensor 324 is coupled to asupply pressure of the valve positioner 306 and is configured to measurethe level of the supply pressure provided to the valve positioner 306.The pressure sensor 324 may provide the supply pressure measurements tothe processor 208. The pressure sensor 324 may be an analog pressuresensor, in which case the output of the pressure sensor 324 may becoupled to an analog to digital converter 325. The analog to digitalconverter 325 may convert the analog signal produced by the pressuresensor 324 to a digital signal suitable for use by the processor 208.Alternatively, the pressure sensor 324 may be a digital pressure sensor.For example, the pressure sensor 324 may include an analog to digitalconverter internal to the pressure sensor 324. In this case, the analogto digital converter 325 may be omitted from the valve positioner 306and the output of the pressure sensor 324 may be provided directly tothe processor 208.

The overpressure protection module 322 comprises computer-readableinstructions stored in the memory 210 and executable by the processor208. The overpressure protection module 322 may be the same as orsimilar to the overpressure protection module 222 of FIG. 2. Theoverpressure protection module 322 may operate in the same or similarmanner to the overpressure protection module 222 to detect an abnormalpressure and, in response to detecting the abnormal pressure, to controlthe level of the drive signal provided to the first pneumatic stage 215so as to limit the output pressure of the valve positioner 306. In theembodiment of FIG. 3, however, the overpressure module 322 operates bydetecting the abnormal pressure based on supply pressure measurementprovided by the pressure sensor 324. For example, the overpressureprotection module 322 may obtain a measurement of the supply pressurefrom the pressure sensor 324 and may compare the obtained supplypressure measurement to a predetermined threshold. The overpressureprotection module 322 may detect an abnormal pressure when the supplypressure measurement exceeds the predetermined threshold. In response todetecting the abnormal pressure, the overpressure protection module 322may control the level of the drive signal provided to the firstpneumatic stage 215. For example, the overpressure protection module 322may set the drive signal to zero milliamperes, set the drive signal tozero millivolts, set the drive signal to another suitable value, preventfurther adjustments of the drive signal, prevent further increases inthe drive signal, etc., as described above with respect to theoverpressure protection module 222 of FIG. 2.

FIG. 4 is a flow chart of an overpressure protection scheme 400 that maybe implemented by the overpressure protection module 222 of FIG. 2 orthe overpressure protection module 322 of FIG. 3. With reference toFIGS. 2 and 3, the overpressure protection module 222 or theoverpressure protection module may operate according to the scheme 400to control the drive signal provided to the first pneumatic stage 215 soas to limit the output pressure of the valve positioner 206. At block402, the overpressure protection module obtains a measurement from apressure sensor, such as a pressure sensor coupled to the output of thevalve positioner or a pressure sensor coupled to a supply pressure ofthe valve positioner. At blocks 404, the overpressure protection modulecompares the pressure measurement obtained at block 402 to apredetermined threshold. If the measured pressure exceeds thepredetermined threshold, then the scheme 400 continues at block 406 atwhich the overpressure protection module control the drive signalprovided to the pneumatic stage of the valve positioner so as to reducethe output pressure of the valve positioner. For example, theoverpressure protection module sets the drive signal to a value at ornear zero milliamperes, set the drive signal at or near zero millivolts,or set the drive signal to any other suitable value. Alternatively, theoverpressure protection module may prevent further adjustments of thedrive signal thereby locking the drive signal at the current value ofthe drive signal. At yet another example, the overpressure protectionmodule may prevent a further increase of the drive signal, while stillallowing a decrease in the drive signal, or may control the drive signalin another suitable manner so as to limit the output pressure level ofthe valve positioner. In any event, the scheme 400 then returns to block402, at which the processor obtains a next measurement from the pressuresensor.

Returning to block 404, if the comparison at block 404 indicates thatthe measured pressure does not exceed (e.g., is less than or equal to)the predetermined threshold, then the scheme 400 simply returns to block402 to obtain a next measurement from the pressure sensor.

FIG. 5 is a block diagram of the field device 200 arranged in accordancewith another embodiment of the present disclosure. In the embodiment ofFIG. 4, the valve positioner 206 (FIG. 2) is replaced by a valvepositioner 506. The valve positioner 506 is generally similar to thevalve positioner 206 of FIG. 2 and includes many like-numbered elementsto the valve positioner 206 of FIG. 2. In the embodiment of FIG. 5,overpressure protection is provided by a hardware module, such ascontrol circuit, coupled to the first pneumatic stage 215 and configuredto control the drive signal provided to the first pneumatic stage 215 soas limit the output pressure of the valve positioner 506 in response todetecting an abnormal output pressure of the valve positioner 506.

The valve positioner 506 includes an overpressure protection module 522coupled between the processor 208 and the pneumatic module 214. Apressure sensor 524 is coupled to the output pressure of the valvepositioner 506 and is configured to measure the level of the outputpressure of the valve positioner 506. The pressure sensor 524 providesthe output pressure measurements to the overpressure protection module522. The overpressure protection module 522 may include analog circuitryand/or digital circuitry configured to detect an abnormal outputpressure of the valve positioner 506 based on output pressuremeasurements provided by the pressure sensor 524. If necessary, thepressure sensor 524 may be coupled to an analog to digital converter, oralternatively to a digital to analog converter, to produce a signalsuitable for use by the overpressure protection module 522.

In response to detecting abnormal pressure based on a measurementobtained from the pressure sensor 524, the overpressure protectionmodule 522 operates to affect the level of the drive signal supplied tothe pneumatic stage 215 so as to limit the pressure output of the valvepositioner 506. For example, the overpressure protection module 522 may,in response to detecting the abnormal pressure, set the drive signal toa level at or near zero milliamperes or at or near zero millivolts (shutoff the drive signal), set the drive signal to another suitable value,prevent further adjustments of the drive signal, prevent furtherincreases in the drive signal, etc., as described above with respect tothe overpressure protection module 222 of FIG. 2.

FIG. 6 is a block diagram of the field device 200 arranged in accordancewith another embodiment of the present disclosure. In the embodiment ofFIG. 6, the valve positioner 206 (FIG. 2) is replaced by a valvepositioner 606. The valve positioner 606 is generally similar to thevalve positioner 506 of FIG. 5 and includes many like-numbered elementsto the valve positioner 506 of FIG. 5. In the embodiment of FIG. 6,overpressure protection is provided by a hardware module, such ascontrol circuit, coupled to the first pneumatic stage 215 and configuredto control the drive signal provided to the first pneumatic stage 215 soas limit the output pressure of the valve positioner 606 in response todetecting an abnormal supply pressure of the valve positioner 606.

The valve positioner 606 includes an overpressure protection module 622coupled between the processor 208 and the pneumatic stage 214. Apressure sensor 624 is coupled to the supply pressure of the valvepositioner 606 and to the overpressure protection module 624. Thepressure sensor 624 is configured to measure the level of the supplypressure of the valve positioner 606 and to provide the supply pressuremeasurements to the overpressure protection module 622. The overpressureprotection module 622 may include analog circuitry and/or digitalcircuitry configured to detect an abnormal pressure based on supplypressure measurements obtained from the pressure sensor 624. Ifnecessary, the pressure sensor 624 may be coupled to an analog todigital converter, or alternatively to a digital to analog converter, toproduce a signal suitable for use by the overpressure protection module622.

In response to detecting abnormal pressure based on a measurementobtained from the pressure sensor 624, the overpressure protectionmodule 622 controls the level of the drive signal supplied to thepneumatic stage 215 so as to limit the pressure output of the valvepositioner 606. For example, the overpressure protection module 622 may,in response to detecting the abnormal pressure, set the drive signal toa level at or near zero milliamperes or at or near zero millivolts (shutoff the drive signal), set the drive signal to another suitable value,prevent further adjustments of the drive signal, prevent furtherincreases in the drive signal, etc., as described above with respect tothe overpressure protection module 222 of FIG. 2.

Referring to FIGS. 5 and 6, although the valve positioners 506 and 606are illustrated as digital valve positioners, the valve positioners 506and 606 may alternatively be analog valve positioners configured toreceive an analog command signal, such as a 4-20 mA command signal, andto control a position of the valve 202 in accordance with the analogcommand signal. In some such embodiments, the interface 212 and/or theprocessor 208 and the memory 210 may be omitted from the valvepositioner 506, 606. In such cases, the analog command signal may beprovided to the pneumatic stage 214 via the overpressure protectionmodule 522, 622 to provide overpressure protection for the actuator 204.

In various embodiments described above, the overpressure protectionmodules 222, 322, 522, 622 may be configured to, in response todetecting an abnormal pressure (e.g., supply input abnormal pressure orcontrol pressure output abnormal pressure), cause a signal indicative ofthe abnormal pressure to be sent to a controller and/or a host devicewithin the process control system of which the field device 200 is apart, such as to the process controller 11 of FIG. 1. Transmitting asignal indicative of the detected abnormal pressure to a controller or ahost device within the process control system may indicate to anoperator monitoring the process control system that the airset 220 hasfailed, and may allow the operator to take an appropriate action, suchas repairing or replacing the airset, shutting down the field device200, shutting down the process control system (or a portion thereof)that includes the field device 200, etc.

FIG. 7 is a flow chart of an exemplary method 700 for limiting controlpressure provided to an actuator of a valve coupled to a valvepositioner. In various embodiments, the method 700 is implemented by thefield device 200 of FIG. 2. In an embodiment, the method 700 isimplemented by the processor 208 in accordance with the overpressureprotection module 222 stored in the memory 210. In another embodiment,the method 700 is implemented using a hardware overpressure protectionmodule coupled to a current drive input of a pneumatic stage of thevalve positioner. In other embodiments, the method 700 is implemented atleast partially using other components of the field device 200 or isimplemented by devices other than the field device 200.

At block 702, a pressure measurement is obtained. In an embodiment, thepressure measurement is obtained from a pressure sensor coupled to acontrol pressure output of the valve positioner. In another embodiment,the pressure measurement is obtained from a pressure sensor coupled to asupply pressure input of the valve positioner. At block 704, an abnormalpressure is detected based on the pressure measurement obtained at block702. For example, the pressure measurement is compared to apredetermined threshold, and the abnormal pressure is detected when thepressure measurement obtained at block 702 exceeds the predeterminedthreshold. Then, at block 706, in response to detecting the abnormalpressure at block 704, a drive signal provided to a pneumatic stage ofthe valve positioner is controlled so as to limit the output controlpressure of the valve positioner. For example, the block 706 may includesetting the drive signal to zero milliamperes, setting the drive signalto zero millivolts, setting the drive signal to another suitable value,preventing further adjustments of the drive signal, preventing furtherincreases in the drive signal, etc., as described above, in variouscontemplated embodiments.

While various functions and/or systems of field devices have beendescribed herein as “modules,” “components,” or “function blocks,” it isnoted that these terms are not limited to single, integrated units.Moreover, while the present invention has been described with referenceto specific examples, those examples are intended to be illustrativeonly, and are not intended to limit the invention. It will be apparentto those of ordinary skill in the art that changes, additions ordeletions may be made to the disclosed embodiments without departingfrom the spirit and scope of the invention. For example, one or moreportions of methods described above may be performed in a differentorder (or concurrently) and still achieve desirable results.

The invention claimed is:
 1. A method of limiting control pressureprovided to an actuator of a valve coupled to a valve positioner, themethod comprising: providing a drive signal to a pneumatic stage of thevalve positioner, wherein the pneumatic stage is arranged to controloutput pressure of the valve positioner in accordance with the drivesignal; obtaining a pressure measurement from a pressure sensorcommunicatively coupled to the valve positioner; detecting an abnormalpressure based on the pressure measurement; and in response to detectingthe abnormal pressure, controlling the drive signal so as to limit theoutput pressure of the valve positioner, wherein the output pressureprovides control pressure to the actuator.
 2. The method of claim 1,wherein the pressure sensor is configured to sense a level of a supplypressure provided to the valve positioner.
 3. The method of claim 1,wherein the pressure sensor is configured to sense a level of the outputpressure of the valve positioner.
 4. The method of claim 1, whereindetecting the abnormal pressure comprises: comparing the pressuremeasurement to a predetermined threshold; and determining that thepressure is abnormal when the measured pressure exceeds thepredetermined threshold.
 5. The method of claim 1, wherein the valvepositioner includes a processor and a memory, and wherein detecting theabnormal pressure and controlling the drive signal comprises executingcomputer readable instructions stored in the memory.
 6. The method ofclaim 1, wherein the valve positioner includes a control circuitconfigured to receive the pressure measurement, and wherein detectingthe abnormal pressure and controlling the drive signal is performed bythe control circuit.
 7. The method of claim 1, wherein the drive signalis a current signal, and wherein controlling the drive signal comprisessetting the drive signal to a value at or near zero milliamperes.
 8. Themethod of claim 1, wherein the drive signal is a voltage signal, andwherein controlling the drive signal comprises setting the drive signalto a value at or near zero millivolts.